In this type of positioner device, it is necessary to retain a piston rod, which has to perform certain functions in machines, devices and control systems of industrial installations, in a selected position, as desired, according to circumstances without the use of fixed stops.
In order to meet these requirements, certain pneumatic operating cylinders with stroke adjustment are known wherein the piston is attempted to be held in a particular stroke position in that the two sides of the piston of the operating cylinder are acted upon simultaneously by opposing air pressures. However, this kind of control for operating cylinders of the above type has the disadvantage that it is not possible to predetermine an accurate holding position of the piston, i.e., the piston always moves to an undetermined holding position which deviates from the position or holding position desired, particularly when there are leaky lines.
As is also known, a stroke adjustment in a double-action, pneumatic operating cylinder can be obtained by means of adjustable stops which act directly upon stem ends of a reversing valve in the form of a two-way valve, in which case the stops execute a movement in synchronism with the piston or piston rod, and the reversing valve is mounted on the cylinder and thus connected therewith in such a manner that the control element of the reversing valve and the piston or piston rod can be displaced in an opposite direction relative to each other. When making use of simple, fixed stops, in particular in larger equipment, there would not only occur much noise but also the control devices would be subjected to forceful impacts, which would further lead to early destruction of individual components.
In the case of braking devices which have to absorb the full piston power, it is further a great disadvantage that when the piston is set in motion again, this happens abruptly. Moreover, with these devices there is a high surface pressure on the piston rod, which leads to move rapid wear and tear.